Pioneering work in the surgical technique of anastomosis of arteries and veins was made in the early 1900s by the French surgeon Alexis Carrel (1873-1944). From Carrel's careful methods of protecting the vessels during harvest and storage and delicate anastomosis operations he laid the groundwork for the development of vascular surgery and transplantation. In 1912, Carrel wrote: “In operations on blood-vessels certain general rules must be followed. These rules have been adopted with the view of eliminating the complications which are especially liable to occur after vascular sutures, namely, stenosis, haemorrhage, and thrombosis.” One such rule was to carefully wash the vessel with Ringer's solution and coat it and the surrounding parts of the operating-field with Vaseline to protect the endothelium against “coagulating blood and the juices of tissues”. Carrel knew that damage to the vascular endothelium, the largest organ of the body, led to injury, thrombus and poor outcomes.
One hundred years later, despite major advances in vascular biology and pathobiology, surgeons are still debating the best way to harvest, store and transplant arterial or venous grafts for vascular or cardiac surgery. Globally there are over 800,000 patients who undergo coronary artery bypass graft (CABG) surgery each year, with more than 350,000 patients in the US. On average there are three grafts per operation or about 2.4 million anastomoses performed globally each year, or about 1.0 million in the USA. With the current technology, the current patency rate of arterialisation saphenous vein (SV) grafts following CABG is 80% in the first year, and the patency at 10 years is around 60% compared to 85% for the left internal mammary (LIMA) grafts to the left anterior descending coronary artery (LAD). Thus, an ongoing problem is the early graft occlusion rate of 20% in the first year.
The reasons for a high occlusion rate in the first year may involve vessel damage, including endothelial damage and alterations in vascular reactivity of the vessel, which may have occurred during: 1) the graft harvest (blunt surgical trauma), 2) stretching the vessel, 3) high pressure testing, 4) storage of the graft 5) surgical attachment, 6) temperature fluctuations, and 7) ischemia-reperfusion injury during restoration of blood flow following the anastomoses and/or during reanimation before removing the patient off bypass. In addition, the allograft graft vessels may have different pre-existing pathologies and wall thicknesses (e.g atherosclerosis, fibrosis, post inflammatory changes, various degrees of varicosis etc), which would impact on the vulnerability to injury and stability of the graft.
A particular area of concern with current technologies is the storage procedure and time between harvest and surgical attachment (or re-implantation), which may for example extend to 5 hours or longer during a CABG operation. The storage procedure includes placing the harvested vessel conduit in a solution which may be a patient's heparinized blood, tissue culture medium, Hanks solution or a crystalloid solution including hyperkalemic cardioplegia. Particular attention must be paid to the storage temperature of the solution which may effect the extent and duration of graft ischemia during harvest and during surgical attachment. In more difficult operations and on older patients, surgeries and storage times may be up to 5 hours before re-implantation.
One of the key strategies in the protection and preservation of the transplant is to prevent the vascular endothelium from becoming injured or activated and to preserve endothelium-smooth muscle interactions. An injured or activated endothelium loses many of its homeostatic or balancing functions and becomes proinflammatory and prothrombolytic, prooxidant, profibrinolytic and proathrogenic. Thus past methodologies have aimed to reduce graft reactivity, patency and early failure by preserving the functional integrity of the vascular endothelium and its interactions between the blood or bathing solution and the smooth muscle layer of the vessel wall. However, no therapy has proven to be clinically successful as evidenced by the high 20% patency failure in the first year. For example, one troubling and continuing problem with harvested and transplanted grafts for CABG and vascular surgery is vasospasm. Vasospasm is defined as an exaggerated hypercontractile response or state of a vessel's smooth muscle to various stimuli which may be precipitated by endothelial dysfunction, shear stresses, smooth muscle calcium hypersensitivity, increased autonomic tone (parasympathetic and alpha-adrenergic receptors) and increased oxidative stress.
In the past 10 years, vasospasm has become particularly challenging with a resurgence of use of the radial artery graft after it was abandoned in the mid-1970s because of a high incidence of vasospasm and a 35% failure rate at 2 years. Arterial grafts are known to have inherent spasticity compared to saphenous veins, because of a thicker layer of smooth muscle and connective tissue, and different endothelial-smooth muscle functions. Arterial grafts possess more pronounced endothelium-dependent relaxation properties to acetylcholine, bradykinin, histamine, substance P and mechanical sheer stress than saphenous veins. In addition, cooling has shown to act as a vasodilator in human internal thoracic arteries, saphenous veins, aorta, coronary arteries, and pulmonary arteries.
It is not known whether protection could be elicited by a form of artificial hibernation-like state for the graft. Natural hibernators possess the ability to lower their metabolic energy demand for days to months. Hibernation, like sleep, is a form of dormancy and helps to keep the animal's metabolic supply and demand ratio in balance. WO00/56145 (U.S. Pat. No. 6,955,814), WO04/056180 and WO04/056181 describe compositions useful to limit damage to a cell, tissue or organ by administering them to a patient in a clinical setting. Selective administration of adenosine A2A receptors has also been proposed in U.S. Pat. No. 6,372,723.